Electric lamp and discharge devices – Cathode ray tube – Shadow mask – support or shield
Reexamination Certificate
2000-08-25
2002-08-27
Patel, VIP (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Shadow mask, support or shield
C313S407000
Reexamination Certificate
active
06441546
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a shadow mask assembly and a color cathode-ray tube (CRT) having the same and more particularly, to a shadow mask assembly having a structure to fix the shadow mask to the frame under application of a tensile force thereto, thereby keeping the mask in a specific shape, and a color CRT equipped with the assembly.
2. Description of the Related Art
Shadow-mask type color CRTs have been used extensively and most popular. In color CRTs of this type, the shadow mask is usually shaped to form part of a spherical surface. Since the shadow mask is fixed in the neighborhood of a face panel made of glass, the face panel needs to be shaped to form part of a spherical surface as well.
In recently years, there has been the tendency that the face panel is shaped to form part of a cylindrical or flat surface. This s due to the fact that the visibility of the color CRTs can be improved. In this case, it is needless to say that the face panel needs to be shaped to form part of a cylindrical or flat surface according to the shape of the mask.
However, as known well, the shadow mask is formed by an extremely thin, rectangular metal plate and has a lot of regularly-arranged small apertures for color selection and thus, its mechanical rigidity is extremely low. As a result, the mask is unable to keep its cylindrical or flat shape without application of any external force. Thus, it is usual that a proper tensile force or two is/are applied to the flat mask in a direction or two when the mask is fixed to the frame, thereby applying a proper tensile force to the mask in a desired direction to keep it in a desired cylindrical or flat shape.
An example of the structure to fix the shadow mask to the frame under application of a tensile force to the mask is disclosed in the Japanese Non-Examined Patent Publication No. 8-77936 published in March 1996, which is explained below with reference to FIG.
1
.
As shown in
FIG. 1
, the prior-art shadow mask assembly
200
is comprised of a pair of curved bars
201
a
and
201
b,
a pair of arms
202
a
and
202
b,
and a rectangular shadow mask
203
. The pair of bars
201
a
and
201
b
are fixed to the pair of arms
202
a
and
202
b,
thereby constituting the frame that supports the mask
203
.
Each of the bars
201
a
and
201
b
has an L-shaped cross section and is curved to form part of a cylindrical surface. Each of the arms
202
a
and
202
b
is bent to have a shape like a U letter. The ask
203
, which is curved to form part of a cylindrical surface, has a lot of regularly-arranged small apertures
204
allowing three electron beams for red (R), green (G), and blue (B) to pass through for color selection.
The two opposite ends of the mask
203
are fixed by welding to the curved top faces of the bars
201
a
and
201
b,
respectively. Thus, the mask
203
covers or closes the rectangular opening formed by the bars
201
a
and
201
b
and the arms
202
a
and
202
b
(i.e., formed by the frame). In this state, the arms
202
a
and
202
b
have elastic forces that translate respectively the bars
201
a
and
201
b
outwardly. Due to these elastic forces, the fixed ends of the mask
203
are pulled so as to be apart from each other. As a result, a tension that translate the fixed ends of the mask
203
to be apart from each other is generated in the mask
203
, thereby keeping the shape of the mask
203
.
According to the Publication No. 8-77936, the bars
201
a
and
201
b
and the arms
202
a
and
202
b
(which constitute the frame) are made of chromium-system stainless steel while the shadow mask
203
is made of iron.
The Publication No. 8-77936 does not disclose any fabrication method of the prior-art shadow mask assembly
200
. However, it is supposed that the assembly
200
is fabricated in the following way.
First, the two ends of the arm
202
a
are fixed by welding to the specific positions of the bars
201
a
and
201
b,
respectively. Similarly, the two ends of the arm
202
b
are fixed by welding to the specific positions of the bars
201
a
and
201
b,
respectively. Thus, the bars
201
a
and
201
b
are apart from each other by a specific distance and arranged in parallel, constituting the frame having an approximately rectangular shape.
Subsequently, the bars
201
a
and
201
b
are applied with an external force or forces to be shifted nearer, thereby slightly bending the ends of the arms
202
a
and
202
b
inwardly. Next, while the ends of the arms
202
a
and
202
b
are kept bent inwardly, the two opposite ends of the mask
203
are respectively fixed to the curved top faces of the bars
201
a
and
201
b
by welding. Thereafter, the application of the external force or forces is stopped, allowing the ends of the arms
202
a
and
202
b
to return to their original positions. Thus, an outward elastic force that moves the bars
201
a
and
201
b
to be apart from each other is generated, applying a specific tensile force to the mask
203
in a direction perpendicular to the bars
201
a
and
201
b.
Due to the specific tensile force, the desired shape of the mask
203
can be kept unchanged in spite of its extremely low mechanical rigidity.
Generally, when the color CRT is operated, large part of he R, G, and B electron beams collide with the mask
203
, raising he temperature of the mask
203
to near 100° C. Due to this temperature rise, the mask
203
is partially expanded thermally to hereby induce undesired positional shift of the apertures
204
. In this state, the R, G, and
5
electron beams tend to land in error on the phosphor screen located between the mask
203
and the face panel, degrading the color purity. This phenomenon is termed the “doming”.
The tensile force applied to the shadow mask
203
serves not only to hold its shape but to prevent the doming phenomenon. Thus, if the mask
203
has a large thermal expansion coefficient, the tensile force needs to be increased according to the value of the coefficient.
With the prior-art shadow mask assembly
200
shown in
FIG. 1
, the s shadow mask
203
is made of iron and therefore, the thermal expansion coefficient of the mask
203
is as large as approximately 120×10
−7
/° C. at room temperature. Thus, to absorb the thermal expansion of the mask
203
and to mechanically hold the desired shape of the mask
203
securely, the necessary tensile force is as high as approximately 500 to 1000 kg. As a result, the bars
201
a
and
201
b
and the arms
202
a
and
202
b
need to be stronger according to the magnitude of the tensile force applied, which raises the weight and fabrication cost of the assembly
200
.
Moreover, a large pressing force or forces need to be applied to the arms
202
a
and
202
b
in the welding process for the mask
203
and therefore, large-scale, expensive facilities are required for this purpose. This makes the fabrication cost of the assembly
200
higher.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a shadow mask assembly that reduces the necessary tensile force to be applied to the shadow mask, and a color CRT equipped with the assembly.
Another object of the present invention is to provide a shadow mask assembly that reduces the weight and fabrication cost of the shadow mask assembly, and a color CRT equipped with the assembly.
Still another object of the present invention is to provide a shadow mask assembly that requires no large-scale, expensive facilities for the purpose of applying the tensile force to the shadow mask, and a color CRT equipped with the assembly.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, a shadow mask assembly is provided, which is comprised of;
(a) a pair of bars arranged in substantially parallel at a specific distance;
the pair of bars being made of metal having a first thermal expansion coefficient;
(b) a pair of arms fixed to
McGinn & Gibb PLLC
NEC Corporation
Patel VIP
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